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Revenue and Related Matters - Disaggregation of Revenue by Geographic Area (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | $ 6,267,411 | $ 5,906,956 | $ 5,475,846
United States and Canada |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | 4,017,730 | 3,911,042 | 3,619,382
Europe, Middle East and Africa |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | 1,517,815 | 1,332,070 | 1,234,659
Other International |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | 731,866 | 663,844 | 621,805
Research |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | 5,125,650 | 4,887,046 | 4,604,791
Research | United States and Canada |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | 3,319,879 | 3,275,866 | 3,056,096
Research | Europe, Middle East and Africa |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | 1,209,660 | 1,060,959 | 1,017,860
Research | Other International |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | 596,111 | 550,221 | 530,835
Conferences |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | 583,224 | 505,164 | 389,273
Conferences | United States and Canada |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | 354,011 | 317,531 | 263,165
Conferences | Europe, Middle East and Africa |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | 165,306 | 140,640 | 88,979
Conferences | Other International |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | 63,907 | 46,993 | 37,129
Consulting |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | 558,537 | 514,746 | 481,782
Consulting | United States and Canada |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | 343,840 | 317,645 | 300,121
Consulting | Europe, Middle East and Africa |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | 142,849 | 130,471 | 127,820
Consulting | Other International |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenues | $ 71,848 | $ 66,630 | $ 53,841

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